Sharing a single VM on multiuser accounts (Ubuntu)

Description

Sorry for the poor english... let's try it :

I'm using Vbox since a few weeks, on ubuntu. I've created 4 accounts on my ubuntu. On the VM, I've winXP PRo installed, with the same 4 accounts.
The VM is installed on a shared directory with full acces rwx to the vbox users, outside the /home/user/.VirtualBox...

Problem :
I wish I can have full access to the VM with all 4 users of ubuntu, but it's not totally possible... Indeed, there's a file which is created (each time you modify something in the VM config) in the user's directory/home/userA/.virtualBox/VirtualBox.xml
When you logout the VM, and the user A session, and login on a userB session & restart the VM, if something has changed on the VM config, an error occurs. I assumed that's why it's impossible to use the snapshot fonction when you have more than 1 user on ubuntu...

To solve the problem, I'm editing the VirtuaBox.xml files, on my 4 account and copy/paste the up-to-date file... But that's pretty boring...

It will be great if we could have only ONE VirtualBox.xml file, in a share place...

possible enhancement 1:
Make possible an install on a share directory, outside the /home/user/.virtualbox/ directory, with ALL the necessary files in it...
possible enhancement 2:
Make possible to use a single .vdi file for 2 different VM... Then, it will be possible to create a different VM for each users of ubuntu, which can access to the same "harddrive".